Alkenes

C=C double bond Formula c2h2n They react in a similar manner They differ by CH2

Bromine water and shake and if it goes colourless then alkanes are present

Electrophilic addition is the addition of an electrophile to a double bond

The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms

Electrophilic addition reactions include the addition of: Steam (H2O (g)) Hydrogen halide (HX) Halogen

The double bond in alkenes is an area of high electron density (there are four electrons found in this double bond) This makes the double bond susceptible to attack by electrophiles (electron-loving species)

A molecule of hydrogen bromide (HBr) is polar as the hydrogen and bromine atoms have different electronegativities The bromine atom has a stronger pull on the electrons in the H-Br bond As a result of this, the Br atom has a partial negative and the H atom a partial positive charge

In an addition reaction, the H atom acts as an electrophile and accepts a pair of electrons from the C=C bond in the alkene The H-Br bond breaks heterolytically, forming a Br- ion This result in the formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile)

Bromine (Br2) is a non-polar molecule as both atoms have similar electronegativities and therefore share equally the electrons in the covalent bond

when a bromine molecule gets closer to the double bond of an alkene, the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged

In an addition reaction, the closest Br atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene The Br-Br bond breaks heterolytically, forming a Br- ion This results in the formation of a highly reactive carbocation intermediate which reacts with the Br- (nucleophile

Gaseous hydrogen bromide

Aqueous bromine

In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond

Carbocations are positively charged carbon atoms with only three covalent bonds instead of four

There are three types of carbocations: primary, secondary and tertiary

The alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’ This is also known as the inductive effect of alkyl groups

The inductive effect is illustrated by the use of arrowheads on the bonds

The alkyl groups push electrons away from themselves towards the positively charged carbon This causes the carbocation to become less positively charged As a result of this, the charge is spread around the carbocation which makes it energetically more stable

tertiary carbocations are the most stable as they have three electron-donating alkyl groups which energetically stabilise the carbocation

Due to the positive charge on the carbon atom, carbocations are electron-loving species (electrophiles)